Balancing risks and rewards of alternate strategies in the seaward extent, duration and timing of fjord use in contemporary anadromy of brown trout (Salmo trutta)

Background Anadromy comprises a successful life-cycle adaptation for salmonids, with marine migration providing improved feeding opportunities and thus improved growth. These rewards are balanced against costs from increased energy expenditure and mortality risk. Anthropogenic-induced environmental changes that reduce benefits and/or increase costs of migration e.g., aquaculture and hydropower, may therefore result in adaptations disfavouring anadromy. We tagged brown trout (Salmo trutta) smolts (N = 175) and veteran migrants (N = 342), from five adjacent riverine populations located in Sognefjorden, the longest Norwegian fjord-system supporting anadromous brown trout populations (209 km). Over four years, 138 acoustic telemetry receivers were deployed to track migrations of tagged individuals from freshwater and throughout Sognefjorden. Detected movements were used to fit migration models and multi-state mark-recapture models of survival and movement for each life-stage. Seaward migration distance was modelled to examine the fitness consequences from alternate migration strategies, with these models used to simulate the extent of fjord-use by individuals and accompanying growth, fecundity and survival consequences. We compared these findings with mark-recapture data collected prior to aquaculture and hydropower development. Results The telemetry data revealed that the outermost-fjord region was utilised by all populations albeit by few individuals. However, historical recaptures were located at a greater distance from the river mouth (87.7 ± 70.3 km), when compared to maximum migration distances of present-day counterparts (58.6 ± 54.9 km). River of origin influenced observed migratory behaviour and differential survival was estimated for each population and life-stage. The simulations based on telemetry-data models revealed a 30% and 23% difference in survival among populations for smolts and veteran migrants, respectively. At the individual-level, a long-distance migration strategy was rewarded with enhanced fecundity. However, the main contribution to population-level fecundity was overwhelmingly derived from middle-distance migrants, due to higher mortality rates and limited numbers of long-distant migrants. Conclusions We conclude that present-day anadromy is precarious, but potential risk varies considerably between life-stages and populations, even within a single fjord system. Our findings suggest that selection for extended migration is under pressure, we therefore stress the importance of monitoring and management actions to secure genetic variation pertinent to preserve fitness gains of anadromy. Supplementary Information The online version contains supplementary material available at 10.1186/s12862-023-02179-x.

Figures are presented and coloured according to tagging river, data is presented for all sampling years (2012 -2015).
Figure S3: Boxplots showing the mean and inter-quartile range of back calculated estimates of first-and second-year sea specific growth rate.(a) Growth rate of Laerdal brown trout sampled during the 'past ' (1956 -1970) and 'present' (2009 -2014) 2 for an overview of the historical and contemporary scale samples from which the 1 st / 2 nd year specific growth rate at sea were estimated and the number of fish which were implanted with an acoustic tag is also stated.Note: Where more than ten models were built, only the top ten performing models are given.
In all instances the principle of parsimony was adhered to and the model with the lowest AIC score was selected.ΔAICc denotes the difference between a candidate model's AICc value compared to the one with the lowest AICc.Total fish length is indicated by TL; daily mean standardised water discharge, stQ and sequential change in daily mean water discharge, ΔQ.Note: Models were selected according to AIC (Table S2).SE denotes the standard error for the estimates, and CI the confidence interval.Note: N denotes number, SD standard deviation.The values were derived from acoustic telemetry data, conducted during a six-month period over three years.(WoY: 13 -40, 2013(WoY: 13 -40, -2015)).Where: ĉ = 1.27,Npar = number of model parameters, Ness = the effective sample size.
ΔQAICc is the difference between a candidate model's QAICc compared to the one with the lowest QAICc.
"/" indicates predictor separation, "+" indicates grouped as a single predictor, "*" indicates interactive effect of predictor and "( .)" indicates that predictors were held constant.Note: SE denotes the standard error for the parameter estimates.UCI/LCI denotes the upper and lower 95% confidence intervals for each parameter estimate.
"/" indicates predictor separation, "+" indicates grouped as a single predictor, "*" indicates interactive effect of predictor and "( .)" indicates that predictors were held constant."/" indicates predictor separation, "+" indicates grouped as a single predictor, "*" indicates interactive effect of predictor and "( .)" indicates that predictors were held constant.Note: Dependant on max zone reached, TL at end of growth season was estimated as   =    * 0.5 , where sea growth was estimated from 2nd sea age specific growth ( 2 ).Size-specific estimates of fecundity (  ̅̅̅̅̅̅̅̅ ) were generated according to   =  −4.03+2.74*   .Only individuals contributing to the spawning population (  > 35 cm and retuned to freshwater during the period WoY 37 -52), were included in the estimates of realised fecundity.

Figure S2 :
Figure S2: Detection data of Sognefjord Salmo trutta veteran migrants (N = 250), sorted by time periods.(b) Contemporary (2012 -2014) growth rate of acoustically tagged Sognefjord veteran migrant brown trout, grouped according to maximum habitat zone reached.(c) Contemporary (2012 -2014) growth rate of acoustically tagged Sognefjord veteran migrant brown trout, dependent on river of origin.No significant difference in growth rate was observed between time periods (a), nor maximum habitat zone use (one-way ANOVA: p > 0.05) (b).A significant difference was observed between rivers for the first-year growth rate at sea (one-way ANOVA: F = 3.698, p = 0.0139) (c), but no significant difference in second-year growth rate was revealed (one-way ANOVA: p > 0.05).derived Note: Refer to Table R 2 gives the adjusted r-squared value of the model.Total fish length is denoted by TL; daily mean standardised water discharge, stQ and sequential change in daily mean water discharge, ΔQ.The seasonal periods were defined accordingly: winter-late = week of year (WoY) 1 -12, spring/summer = WoY 13 -26, autumn = WoY 27 -40 and winter-early = WoY 41 -52.

Figure S5 :
Figure S5: Total residence duration (in weeks) of individual tagged brown trout smolt

Figure S6 :
Figure S6: Mean annual depth use of individual veteran migrant brown trout.Depth data was

Figure S7 :
Figure S7: (a) Simulated trajectories of habitat use for 1000 individual veteran migrant

Figure S8 :
Figure S8: Simulated estimates of (a) individual and (b) population fecundity (N of eggs) of Sognefjorden veteran migrant brown trout, dependent upon selection of an individual's maximum migration extent (fjord habitat zone: inner-, mid-or outer-fjord) and their river of origin, presented for 2013 -2015 conditions.Realised individual fecundity is estimated from the product of average expected size-specific fecundity (  ̅̅̅̅̅̅̅̅ ) and the maximum zonespecific survival rate   .Realised population fecundity is estimated from the product of total (  ̅̅̅̅̅̅̅̅ ) and mean survival (  ).Only individuals predicted to contribute to the spawning population are included in the realised estimates of egg numbers (from the initial population of 1000).

Table S1 :
Overview and technical specifications of the acoustic tags deployed in smolt (a) and veteran migrant (b) brown trout from Sognefjorden.All tags were produced by Thelma Biotel AS, Trondheim, Norway.

Table S2 :
AIC scores of the models generated to describe the migration and habitat use of tagged Salmo trutta smolts and veteran migrants in Sognefjorden.

Table S3 :
Summary statistics from the models used to describe the migration and habitat use of tagged brown trout smolts and veteran migrants in Sognefjorden.

Table S4 :
Summary statistics of residence duration (in weeks) within each given habitat zone of tagged Sognefjord brown trout smolt migrants over a six-month period.

Table S5 :
Summarised seasonal depth use of veteran migrant brown trout within each given habitat zone of Sognefjorden.
Table 4(b) states mean values of realised individual and population fecundity for each study population.